Thermal power conversion using fossil fuel is a technology used by major power utilities to generate electricity. Thermal power cycles typically convert chemical energy of fossil fuels into thermal energy of flue gas through combustion. The thermal energy from flue gas is transferred to pressurized water-steam which drives turbines through steam expansion, releasing the remaining thermal energy of the exhaust steam to the ambient environment through condensation, with repressurization of the condensed water for the next cycle. Such energy conversion cycle with water-steam as a working fluid in turbines is known as steam Rankine cycle (SRC). Although water-steam is the predominant working fluid for conventional power cycles, it is possible to use other working fluids as well. Other common working fluids are organic fluids. The energy conversion cycles that employ organic working fluids in turbines are known as organic Rankine cycles (ORCs).
Unfortunately, fossil fuels are also the main source of pollutants; greenhouse gases as well as criteria air contaminants The low efficiencies of conventional steam Rankine cycle systems exacerbate the problem with these emissions. Increasing cycle efficiency is widely viewed as the most effective means to reduce all these emissions.
In general, energy losses are inevitable in the course of any Rankine cycle's operation. Over the past century, there has been a steady increase in steam Rankine cycle efficiency. Approaches to improve SRC efficiency have included: (1) reheat, (2) regeneration, (3) increase of the maximum steam working fluid operating temperature and pressure, (4) decrease of heat sink temperatures by increasing condenser vacuum, (5) recovery of energy losses, and (6) increase of the equipment efficiencies.
These improvements have resulted in the so-called reheat regenerative steam Rankine cycle (RRSRC) and cycle efficiency improvement from 20% to about 40%. This means that even with the best steam Rankine cycles available today, there are still losses of 60%, and as a result, SRC efficiencies remain low.
For example, with conventional reheat and regeneration approaches, as represented by a typical reheat regenerative steam Rankine cycle (RRSRC) system shown in FIG. 1 and a corresponding temperature-entropy (T-S) diagram as shown in FIG. 1A, process 6-7 represent reheat (reheater 58 in FIG. 1), processes 1c-1b-1a, 2c-2b-2a, 3c-3b-3a, 4c-4b-4a, and 5c-5b-5a represent regeneration (feedwater heaters 55b, 55a, 53c, 53b, 53a in FIG. 1).
In regeneration, steam extraction at point 1c releases heat through process 1c-1b-1a to feedwater at point 2a and the feedwater temperature rises up to point 1a; steam extraction at point 2c releases heat through process 2c-2b-2a to feedwater at point 3a and the feedwater temperature rises up to point 2a, and so on. It can be seen that extraction points 1c, 2c, 3c, 4c and 5c are in the superheated region: the higher the temperature of the point 5 and 7, the higher the extraction temperature of points 1c, 2c, 3c, 4c and 5c will be giving a higher temperature difference between the extraction steam and feedwater, which results in greater irreversibility or exergy loss of the regeneration process.
With the conventional approach of decreasing the heat sink temperatures, in FIG. 1A, process 8-1 represents condensation of exhausted steam (condenser 56 in FIG. 1) and exhausted heat 36a in FIG. 1 is discharged to ambient (heat sink) through the cooling water. As steam begins to condense at 100° C. at atmosphere pressure, in order to make the condensation of steam happen at a temperature much lower than 100° C., the condenser must operate at considerably lower pressures. Even if a heat sink source with very low temperature exists such as cooling water in winter or deep lake water (with a temperature of near 4° C. year round below approximately 20 m), the condensing temperature will not be sufficiently low to take full advantage of the cooling water. This is because the volumetric flow rate of steam, under such vacuum conditions, is so high that it results in huge dynamic losses in the turbine and actually lowers the turbine efficiency; meanwhile, the steam becomes very wet in the course of its expansion prior to reaching the condenser. Water droplets in the wet steam will cause serious erosion of turbine blades and will result in safety issues. Of the 60% heat losses in SRC, the majority are due to exhausted heat discharged to the heat sink by steam-water condensation, which is unrecoverable energy loss.
One method of identifying whether losses are recoverable is to compare the real SRC efficiencies with their Carnot cycle efficiencies. For instance, a system with a turbine entry temperature of 565° C. and condenser temperature of 10° C. gives a theoretical Carnot efficiency of about 66%, but the actual cycle efficiency is about 40%, which means that an increase in efficiency of about 26% may still be attainable overall in theory, but not feasible with water-steam as the working fluid.
With respect to the conventional approach of recovery of energy losses, there exist low temperature heat losses 36 in FIG. 1 from the boiler exhaust flue gas. The magnitudes of these losses are sufficiently large that they should not be neglected and opportunities for heat recovery must be sought. Again, these losses are less suitable for recovery with water-steam as the working fluid.
Rankine cycles using working fluids other than water/steam are known.
For example, International patent application WO2009/098471 generally discloses a method and apparatus for generating power wherein water is heated to generate wet steam in a positive displacement steam expander. The expanded steam is condensed and returned to the boiler. The expanded steam may be condensed in the boiler of an Organic Rankine Cycle to provide additional power.
United States Patent Application 20110113780 discloses a waste heat recovery system using a Brayton cycle system as a top cycle circulating carbon dioxide vapour, and a Rankine cycle system as a bottom cycle configured to circulate a working fluid in heat exchange relationship with the carbon dioxide vapor, wherein the working fluid from a heat exchanger is divided into two portions, with one portion being fed to another heat exchanger and subsequently mixed with the other portion of the working fluid.
International patent application WO2009/045117 discloses a power plant having two working cycles thermally coupled with one another by at least one heat exchanger, whereas the working fluid in the lower cycle is a substance with a low evaporation enthalpy and a relatively high preheating enthalpy, preferably an organic fluid, while a high-temperature source of heat is used to preheat, evaporate and superheat the working fluid in the upper cycle.
The above prior art systems and other systems such as those disclosed in U.S. Pat. Nos. 7,287,381, 7,096,665, 7,942,001 and 7,891,189 generally follow the above conventional configurations and/or approaches to SRC efficiency improvement, and inherit the disadvantages associated with those approaches.
Therefore, there remains the need to improve the efficiency of thermal power conversion based on Rankine cycles.